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The Coolest Brown Dwarfs Proliferate

CHICAGO--By collecting and cataloging hundreds of millions of celestial objects, the Sloan Digital Sky Survey may turn a rare oddball into a common denizen of the heavens. The $80 million project, although still in a shakedown phase, has demonstrated that its census-taking power extends to our cosmic neighborhood as well. At an American Astronomical Society meeting here today, survey members announced that it has turned up two of the coolest, dimmest stars called brown dwarfs ever seen.

Probably located within 30 or 40 light-years of Earth, the brown dwarfs are so small that their surface temperature is no more than 1000 Kelvin, cool enough for methane and water--compounds normally associated with planets--to survive as superheated gases. The find is also a hint that, as theorists have predicted, large numbers of such stars are waiting to be found. "Given that the sky survey was built for other purposes, that's a really handsome payoff," says Alan Boss, an astrophysicist at the Carnegie Institution of Washington.

Astronomers stumbled across the brown dwarfs while searching for new quasars in data gathered during a test period of Sloan's special wide-field telescope on Apache Peak, New Mexico. Because quasars are so distant that their light is shifted toward the red end of the spectrum, they glimmer in the Sloan's long-wavelength, infrared channels but vanish in optical bands. So do brown dwarfs, as Michael Strauss and Xiaohui Fan of Princeton University and Zlatan Tsvetanov and Wei Zheng of Johns Hopkins University found when detailed studies of the objects' spectra revealed that they were cool, dim, and close by.

Defined as stars with less than about 8% of the mass of the sun (or 80 Jupiter masses), brown dwarfs never ignite much fusion burning in their cores and gradually dim after they form. Dozens of garden-variety brown dwarfs, called L dwarfs, slightly too massive and warm to host water and methane, have turned up over the last couple of years. But only one methane dwarf had been found before.

As the Sloan expands its view to cover much of the northern sky, the smallest dwarfs could become a tribe. "They're oddballs at the moment," says David Golimowski of Johns Hopkins and the Sloan team, "but I'm confident they'll be pretty boring objects within a few months."